The war was initiated by Sauron, who had gained strength since the end of the Second Age and sought the One Ring he had forged and into which he had invested much of his power, and that he had lost in the climactic battle at the end of the prior age.

The war ended after the Battle of Bywater and, shortly afterwards, the deaths of Saruman and Gríma Wormtongue. Towards the end of the War of the Ring, Elessar was crowned King of Gondor, and forgave the Men who had fought under Sauron, heralding a great renewal of cooperation and communication between Men, Elves, and Dwarves.

The objective of Sauron's grand strategy was to defeat the strongest of the nations that opposed him, Gondor, and to do so he would need to take the capital city and greatest fortress of Gondor, Minas Tirith. To this end, the war effort of Mordor was focused in the south in and around Gondor, in a strategy of divide and conquer. To keep Gondor's ally Rohan, on its northern border, from sending aid, Sauron promoted the rise of Saruman at Isengard to the west of Rohan. Thus all of Rohan's forces would be focused in the west trying to stem the tide of the Isengard attack, and none would be sent to Minas Tirith's defence. Meanwhile, Sauron sent the Mordor-allied Corsairs of Umbar to attack Gondor's populous southern coastal fiefs, which as a result sent only a fraction of their forces to defend Minas Tirith in northern Gondor, while the rest stayed on the coasts preparing for the Corsair assault. However, Sauron's divide and conquer strategy was ultimately foiled and a united front of Gondor and Rohan's forces faced Mordor.

Although there had never really been peace, the War of the Ring started in Rohan when Saruman's troops crossed the Fords of Isen. Théodred, the son of the KingThéoden, had mustered his forces on the fords in order to launch a surprise attack against the enemy. On 23 February T.A. 3019 he attacked the vanguard of the orcs marching out of Isengard. Reinforcements were quickly sent from Isengard however, and Théodred ordered a retreat. His forces retreated to an island in the fords, but they were soon surrounded and he was killed. Grimbold managed to hold the island, but would not have succeeded if Elfhelm had not come with reinforcements from Helm's Deep. Thus the first battle of the Fords of Isen ended in defeat for the Rohirrim.

Now that the Marshal of the West-mark was dead, Erkenbrand took command of the Westfold. He placed Grimbold and Elfhelm at the Fords. However, they were unable to withstand the force of Isengard, and were surrounded. Though they successfully broke through the enemy's lines, they ended up scattered around the Westfold, giving Saruman clear passage into Rohan.

Meanwhile, Gandalf drove Gríma Wormtongue out of Edoras and went to gather Erkenbrand's scattered forces, advising King Théoden to move to the stronghold of Helm's Deep. The king and his forces arrived undisturbed, but soon the fortress was besieged by Saruman's troops. All through the night of March 3–4 a combined force of Orcs and Wild Men from Dunland besieged Helm's Deep, and despite the efforts of the Rohirrim (aided by Aragorn, Legolas and Gimli) hope appeared lost. Believing Rohan was lost, Théoden decided to mount a final, suicidal charge against Saruman's forces. Unexpectedly, however, Gandalf arrived in the nick of time with Erkenbrand and the scattered Rohirrim, along with a forest of Huorns who had been sent to the battle by the Ent leader Treebeard. The orcs were trapped and utterly annihilated. The wild Men taken captive, however, were freed after swearing an oath of loyalty to Rohan and clearing the battlefield of the dead. (The mercy of this act amazed the captives, who had been told by Saruman that the men of Rohan were cruel and burned their captives alive.)

Days before, at an Entmoot in Fangorn Forest, the Ents, furious over the destruction Saruman had wrought upon the forest, had begun marching on Isengard, intending to destroy it with Merry and Pippin. Their sheer strength allowed the Ents to defeat Saruman by March 3. Treebeard then took control of Isengard. At Gandalf's request he sent a large herd of Huorns to the Battle of Hornburg, to aid the Rohirrim.

With the threat of Saruman eliminated, Rohan was able to muster a large force to come to the aid of Gondor.

For over 3,000 years, the realm of Gondor held back the shadow and the threat from the East. In time, the kingdom declined, and Sauron prepared to swoop in for the kill. Faramir, captain of Gondor and son of Denethor, Steward of Gondor, had divided his forces to many fronts. The two most important of these were in the island citadel of Cair Andros and the ruined city of Osgiliath. In a strict sense, the War of the Ring began with the Great Signal from Minas Morgul and the answering signal from Mount Doom, and thus the attack on Osgiliath was the first battle of the war proper. Sauron's two armies obliterated the fortresses, and Faramir was forced to retreat to the causeway forts, the last defence against the Morgul forces. Soon these too were destroyed, and only Minas Tirith remained. The siege soon began, as the Nazgûl hovered above and spread terror and confusion, and siege towers tried to take the walls but were all destroyed. Finally, the Gate of Minas Tirith, which had never before been breached, was broken by a mighty ram, and the Lord of the Nazgûl entered, the first and only enemy to do so. All seemed lost – until six thousand Rohirrim, under King Théoden's command, came and somewhat relieved the Gondorian defenders, but at the cost of many lives, including Théoden's. However, he was avenged with the death of the Witch-king at the hands of Éowyn (aided by Meriadoc Brandybuck). Still, the battle was in doubt until Aragorn arrived with a large force out of South Gondor. The joint force of Gondor and Rohan then successfully defeated their enemies.

The Battle of the Morannon was the final major battle against Sauron in the War of the Ring, fought at the Black Gate of Mordor. The Army of the West, roughly 6,000 strong, led by Aragorn marched on the gate and faced a vastly larger force as a diversionary feint to distract Sauron's attention from Frodo and Sam, who were carrying the One Ring through Mordor. It was hoped that Sauron would think Aragorn had the Ring and was now trying to use it to overthrow Mordor. Despite the seemingly impossible odds the Army of the West was eventually victorious, when the Ring was destroyed, and Sauron's forces fled or surrendered in dismay.

Mordor's war effort was focused in the south against Gondor, but using his outstretched right arm Sauron attempted to flank the lands of the Free Peoples through the north, using Orcs and allied barbarian nations of Men. In this northern theatre of the war (which had spread far across Middle-earth) Sauron's primary objective was to use the forces at his primary base of operations in the areas, Dol Guldur in southern Mirkwood, to defeat Lothlórien, then pass the Misty Mountains (attacking Rivendell), and wheel around to take Rohan and Gondor from the rear. However, Dol Guldur had to deal with the threat of the Woodland Realm of Thranduil, and thus split their forces between the attack on Lothlórien and the one on the Woodland Realm. Sauron wanted to use his barbarian Easterling allies in a joint attack with the Orcs from Dol Guldur on the Woodland Realm, and then have this victorious army link up with the other ones attacking Lothlórien and defeat it. However, unfortunately for Sauron a strong Dwarf nation now existed at the Lonely Mountain thanks to the efforts of Gandalf, as well as the Dwarves of the Iron Hills and allied Men of Dale. Mordor's Easterling allies were tied up fighting the Dwarves of Erebor and Men of Dale, and never linked up with the Mordor forces assaulting the Woodland Realm, which in turn could not link up with those attacking Lothlórien, and the line held.

Bridging the gap between the northern and southern theatres of the war was the line of the River Anduin between Lothlórien and Gondor, running along the Rohan border. Orc armies peeling off from the assault on Lothlórien tried to enter Rohan via this route, while almost its entire army had left to fight at Minas Tirith, but the Ents of Fangorn forest counterattacked and drove the Orcs back in a panic, and most if not all drowned while attempting to flee by crossing the river.

As the war began, the Dwarves of Erebor refused to co-operate with Sauron in his hunt for the Ring. Therefore, Sauron sent an army of Easterlings to Dale. On March 17 they met the armies of the Dwarves and the Men of Dale. After three days of fighting, Men and Dwarves were overrun and sought refuge in Erebor. King Brand of Dale fell before the gate of Erebor, and King under the Mountain Dáin II Ironfoot fell as he was defending Brand's body. Many Men and Dwarves made their escape to Erebor however, and were able to withstand the siege of the mountain fastness. When news spread about the victory in the South, the Easterlings scattered and the sons of Brand and Dáin led their army out of Erebor.

On March 11 Lothlórien was first attacked from Dol Guldur. It was attacked two further times, on the 15 and the 22. When the Dark Lord had fallen, Celeborn led his army out of Lórien, and crossed the Anduin. Dol Guldur was captured and destroyed by Galadriel.

Thranduil of Mirkwood was also attacked from Dol Guldur, but in this, the Battle under the Trees, the Elven folk won a hard victory. After the destruction of Dol Guldur, Celeborn met Thranduil on 6 April, and as the shadow had passed, they divided Mirkwood and renamed it Eryn Lasgalen.

After the defeat of the Dark Lord, Saruman convinced Treebeard, possibly using what little enchantment was left in his voice, to release him from Isengard. He travelled to the Shire, where he replaced Lotho Sackville-Baggins as the Chief under the name Sharkey. Under his command ruffians entered The Shire and ruined it. They were defeated by Hobbits under the lead of Meriadoc Brandybuck and Peregrin Took (Merry and Pippin) in the Battle of Bywater on 3 November. The Hobbits headed to Hobbiton where Frodo ordered Saruman and Wormtongue to leave the Shire. Wormtongue however killed Saruman, before he himself was killed by the Hobbits and their arrows. With the death of the wizard Saruman, the War of the Ring finally ended.

The War of the Ring had great influence on all of the lands of the northwest of Middle-earth. Most importantly it led to the restoration of the Kingdoms of Arnor and Gondor as the Reunited Kingdom, under King Elessar Telcontar (Aragorn). The Steward of Gondor was dead, but he was succeeded by his son Faramir, who kept the office and was given the title Prince of Ithilien.

In Rohan the heir apparent, Théodred, was killed, and, during the Battle of Pelennor Fields, King Théoden died as well. He was succeeded by his nephew Éomer. In the Glittering Caves at the Hornburg, a Dwarven colony was established, and Isengard was given to the Ents to be renamed the Treegarth of Orthanc.

In Dale, both King Brand and King Dáin II Ironfoot were killed, and were succeeded by their sons Bard and Thorin III Stonehelm. They sent their emissaries to the crowning of Elessar, and were in alliance with Gondor until their Kingdoms ended.

For the Elves, the final decline had begun. The bearers of the Three Rings left Middle-earth, and Lórien was eventually abandoned. The Elves of Lothlórien who did not depart over the Sea moved east to the southern third of Mirkwood, below the Narrows, which they named East Lórien. In Eryn Lasgalen however, Thranduil's rule continued, and they had peace. There was also an Elven colony in Ithilien. Many of the Elves of Rivendell departed over the Sea, and by Aragorn's death 120 years later, it was entirely deserted.

In the Shire, life continued as it had prior to the war. It was declared a free land under the Sceptre of Annúminas, and Men were forbidden to enter it. The Westmarch was added to the Shire by King Elessar in 1452 S.R.

Once the Fourth Age began the Elves stopped having an active influence on the affairs of Middle-earth, having started fading away ever since the Third. The Orcs never became a serious threat again; reduced to small bands of mountain brigands, they were never more than a nuisance.[citation needed] The Nazgûl perished with the unmaking of the One Ring and the subsequent loss of the powers of the Nine, while their master Sauron was rendered impotent and unable to threaten Middle-earth again. However, the forces of Rhûn and Harad continued to war with the free peoples of Middle-earth until their eventual decline and defeat.

The War of the Ring is the title of the eighth volume of The History of Middle-earth. The War of the Ring was the working title of "Book 5" of 'The Lord of the Rings' (the first half of the eventual third volume). J. R. R. Tolkien wanted to use this as the overall title for the third volume of The Lord of the Rings, feeling that the title "The Return of the King" gave away too much of the plot.